1. Field of the Invention
This invention relates to a semiconductor light emitting device and, particularly, to a semiconductor light emitting device that a transparent conductive film is used as a current spreading layer. Herein, a semiconductor light emitting device includes a light emitting diode or LED.
2. Description of the Related Art
Conventionally, most of light emitting diodes (herein referred to as LED) as a semiconductor light emitting device had been green LED's of GaP and red LED's of AlGaAs. However, in recent years, GaN-based and AlGaInP-based high quality crystals can be grown by MOVPE (metalorganic vapor phase epitaxy). Thereby, high-brightness LED's with an emission color of blue, green, orange, yellow and red can be fabricated.
An epitaxial wafer fabricated by MOVPE allows LED to have a short-wavelength emission or a high brightness that was not developed thus far. However, in order to obtain an LED with a high brightness, the current spreading property needs to be improved so as to allow current to be uniformly flown into the chip surface of LED. For example, in an AlGaInP-based LED, its current spreading layer needs to have an increased thickness of 5 to 10 μm. Since the material cost for the growth of the current spreading layer is increased, the manufacturing cost of LED must be increased. Thus, this prevents the cost reduction of the AlGaInP-based LED.
Japanese patent application laid-open No. 8-83927 discloses a method that a metal oxide such as ITO (indium tin oxide) and ZnO (zinc oxide) is used as the current spreading layer while having a sufficient light-transmitting property and an electrical property such as a good current spreading property.
A transparent conductive film comprising the metal oxide can have a sufficient current spreading property even in small thickness since its carrier concentration is so high. When the ITO film is used as the current spreading layer, the semiconductor current spreading layer with the increased thickness used conventionally need not be grown. Therefore, the manufacturing cost of the LED or LED epitaxial wafer can be reduced.
However, in case of using the ITO film as a window layer, a contact resistance is generated between the semiconductor layer and the ITO film of metal oxide. Therefore, the forward operation voltage must increase. Namely, the ITO film as a transparent conductive film (or transparent electrode) is of an n-type semiconductor, and an upper contact layer in contact with the ITO film is of a p-type semiconductor. Thus, when a forward operation voltage is applied to the LED, a reverse bias is generated between the transparent conductive film (transparent electrode) and the p-type cladding layer. As a result, little current flows therebetween.
In order to solve the problem, it may be assumed that a thin contact layer with a high carrier concentration is formed in contact with the ITO film so as to operate the LED at a low voltage by tunnel junction. For example, the contact layer may be an As-based contact layer doped with a p-type dopant at a high concentration of 1×1019/cm3 or more.
However, the contact layer needs to be composed such that it generates the tunnel junction and it does not absorb light emitted from the active layer as much as possible. Therefore, the contact layer needs to be thinned while having a high carrier concentration. Because of this, the contact layer is likely to generate the diffusion of dopant due to heat etc. in the crystal growth. The diffusion of p-dopant in the contact layer causes the next two problems.
(i) The first problem is to cause a reduction in light output of the LED. The p-dopant is concentration—diffused in the depth direction of the LED from the contact layer. If it is diffused into the active layer, it becomes a defect in the active layer. The defect composes nonradiative recombination center. As a result, the light output of LED lowers.
(ii) The second problem is to increase the drive voltage (forward operation voltage) of LED. Due to the diffusion of p-dopant, the carrier concentration of the contact layer as a thinned and high-carrier concentration layer substantially lowers. Therefore, the tunnel junction is difficult to generate. Thus, since the tunnel voltage increases, the drive voltage of LED increases.